Corpus callosum (left half)
Corpus callosum
tract
The largest commissural fibre tract of the brain, comprising roughly 200 million axons in the adult human. Conventionally divided rostral-to-caudal into rostrum, genu, body (sub-divided into anterior, middle, and posterior thirds), isthmus, and splenium. The splenium overhangs the pineal recess and the colliculi of the midbrain.
The corpus callosum connects homologous regions of the two cerebral hemispheres in a roughly topographic arrangement: the genu carries prefrontal fibres, the anterior body carries premotor and supplementary motor fibres, the middle body carries motor and sensorimotor fibres, the posterior body carries parietal association fibres, and the splenium carries occipital and inferior temporal fibres. Witelson's 1989 partition scheme remains widely used to divide the midsagittal cross-section into seven such regions.
Most callosal fibres connect homotopic territories, although heterotopic connections also exist, particularly between supplementary motor area on one side and the precentral gyrus on the other. The relative proportions of fast-conducting large-diameter myelinated axons versus thin axons varies along the rostro-caudal axis, with the densest large-diameter populations carrying sensorimotor information across the body.
motor language
Roger Sperry’s and Michael Gazzaniga’s split-brain work in the 1960s and 1970s, on patients whose corpus callosum had been surgically transected to control intractable epilepsy, gave neuroscience some of its most striking experimental confirmations of cerebral hemispheric specialisation. A picture flashed to the right visual field (and thus the left hemisphere) could be named; the same picture flashed to the left visual field (right hemisphere) could not be named verbally, but could be selected by the left hand from a hidden array of objects. The implication, that the disconnected right hemisphere had its own intact-but-mute cognition, motivated decades of subsequent work on hemispheric asymmetry.
The corpus callosum develops late in foetal life and continues to mature into the third decade, with the splenium myelinating last. Agenesis of the corpus callosum is a relatively common developmental malformation that can occur in isolation or as part of syndromes such as Aicardi syndrome and septo-optic dysplasia.
The HRA mesh splits the corpus callosum into left and right halves along the midline. In the rendered atlas these halves can be selected independently, although their fibres of course traverse from one hemisphere into the other across that midline.